1. Field of the Invention
The invention relates to a speed ratio control device of a continuously variable transmission for a vehicle, and in particular to a technology for curbing change of the speed ratio and maintaining a braking effect during ABS operation caused by a braking operation on the vehicle, and also maintaining re-acceleration performance by changing the speed ratio of the continuously variable transmission to the largest possible reduction ratio.
2. Description of Related Art
In a vehicle having a continuously variable transmission and an antilock brake control device (ABS), if a speed changing operation for changing the speed ratio of the continuously variable transmission toward the largest speed ratio before the vehicle is stopped is performed, during operation of the antilock brake control device for intermittently braking the wheels so as to increase the braking force on a low μ road, the engine brake force increases, and the braking operation of the ABS may be affected by the increased engine brake force.
To deal with the above situation, control for fixing the speed ratio of the continuously variable transmission when slipping of a driving wheel is determined, control for immediately changing the speed ratio of the continuously variable transmission toward the largest speed reduction ratio when the deceleration of the vehicle becomes equal to or larger than a first threshold value, and immediately inhibiting change of the speed ratio toward the largest speed reduction ratio when the deceleration becomes equal to or larger than a second threshold value, or control for inhibiting the speed ratio of the continuously variable transmission from being larger than a given value when the antilock brake control device is in operation, has been proposed. For example, technologies described in Japanese Patent Application Publication No. 04-365645 (JP 04-365645 A) correspond to one or more of the above controls.
According to a speed ratio control device of a vehicular continuously variable transmission described in JP 04-365645 A identified above, the engine brake force is less likely or unlikely to increase when the ABS is in operation, thus assuring the braking operation of the ABS.
In the meantime, according to the speed ratio control device of the vehicular continuously variable transmission of the related art, even in the case where the antilock brake control device operates to bring the vehicle into a sudden stop, the speed ratio of the continuously variable transmission may not reach the largest reduction ratio, and sufficient starting response may not be obtained due to a shortage of driving force when the vehicle is re-started.